With the advancement of microfabrication technology, semiconductor integrated device density is continuing to increase, and in particular for Dynamic Random Access Memory (DRAM) arrays. In light of the reduction in device dimensions, the charge storage capability of the capacitors in DRAM cells has become an important issue. The minimum requirements for charge storage, charge sensing, and refresh rate for practical memory applications often results in the cell capacitor being one of the largest DRAM circuit components. Thus, the drive to produce higher capacity DRAM circuitry has give risen to much capacitor development. Moreover, the increasing complexity of VLSI fabrication often entails the use of multilayer structures above the silicon substrate. Therefore, electrical and metallurgical properties of multilayer structures have an important effect on the circuit performance and reliability.
A DRAM basically comprises a multiplexed array of charge storage cells, each cell comprising a capacitor connected to an access device such as a field effect transistor. In such designs, the capacitor electrode which is connected to the access device is generally called the "storage node" or "storage poly" since the material out of which it is formed is often conductively doped polysilicon. The capacitor counter electrode, usually connected to a reference voltage, is called the "cell plate electrode" or "cell poly plate" as it is also often comprised of doped polysilicon. Polysilicon used for capacitor electrodes is typically moderately doped at densities greater than about 10.sup.19 cm.sup.-3 for good conductivity.
In practice, roughened or texturized polysilicon layers are used to increase the effective surface area of the capacitor plates, thereby achieving higher stored charge per unit area of the underlying substrate. To construct such a storage cell, a conductively doped first layer of polysilicon (poly or polycrystalline silicon) is deposited on a wafer. Thereafter, a second polysilicon layer may be deposited and formed into roughened, so-called Hemispherically-Grained Silicon (HGS) thereby forming a capacitor plate having a textured surface morphology. A capacitor dielectric is typically conformally deposited on the HGS poly surface, with the roughened electrode surface providing an enhanced capacitor area. Capacitor dielectrics may for example comprise silicon dioxide or nitride or compositions thereof formed by chemical vapor deposition processes at temperatures ranging from 600.degree. C. to 1000.degree. C. A subsequent polysilicon deposition on the dielectric film forms the second capacitor electrode.
A common problem associated with the aforementioned capacitor fabrication process is the lack of sufficient out-diffusion of dopants from the doped poly into the HSG during subsequent high temperature processing, resulting in a depletion region adjacent to the cell dielectric. The effect of this depletion region introduces an additional "depletion" capacitance in series with the dielectric capacitance, which causes a substantial reduction in the effective capacitance of the storage capacitor.
Clearly, as the lateral dimensions of DRAM capacitors are reduced, performance must increase to meet minimum charge storage criterion. Thus, there is a clear need in the industry to increase DRAM cell capacitance by reducing the effects of dopant depletion.